Some patience required. I have been there ahead of the appointed time, but I couldn't find a booth or else in any of the buildings. However I was able to detect a couple of cars from a rental company (not Sixt) in the parking slots in the center of the pier.
15 min after the appointed time I needed to contact Sixt, because no one showed up. About 15 min later an agent was contacting me by Whatsapp, that he will arrive within the next 5 min. It turned out that one of those cars bearing a logo of a local dealer was mine. Both handover and return took place at the parking slot on the pier and had been without problems. This includes my national driver license from an EU country. The car was in good order, too. So I recommend to contact your agent by Whatsapp, etc. latest at time of arrival. Obviously Sixt is running that delivery service from their airport office about 15 min away, and no one there is keen to wait for late clients.